This invention relates generally to semiconductor packaging and specifically to a method for fabricating BGA packages using a substrate having a patterned solder mask with an open die attach area.
One type of semiconductor package is referred to as a BGA package. BGA packages were developed to provide a higher lead count, and a smaller foot print, than conventional plastic or ceramic semiconductor packages. A BGA package includes an area array of solder balls that permit the package to be surface mounted to a printed circuit board (PCB) or other electronic component.
One type of prior art BGA package 10 is illustrated in FIG. 1A. The BGA package 10 includes a substrate 12, a semiconductor die 16 mounted to the substrate 12, and an encapsulating resin 38 which encapsulates the die 16. As shown in FIG. 1B, the substrate 12 is initially a segment 32 of a panel 30. The panel 30 is similar to a lead frame used in the fabrication of conventional plastic semiconductor packages. The panel 30 includes multiple substrates 12 and is used to fabricate multiple BGA packages 10. Following the fabrication process for the BGA packages 10, the panel 30 is singulated into individual BGA packages 10.
Typically, the substrate 12 comprises a reinforced polymer laminate material, such as bismaleimide triazine (BT), or a polyimide resin. As shown in FIG. 1A, the substrate 12 includes a planar die attach surface 22. During a die attach step of the fabrication process, the die 16 is adhesively bonded to the substrate 12 using an adhesive layer 34.
In addition to the die attach surface 22, the substrate 12 includes an opposing conductor surface 24 wherein conductors 18 are formed in a required pattern. An opening 26 in the substrate 12 provides access for wire bonding wires 28 to the conductors 18, and to bond pads (not shown) on the die 16. In the type of BGA package 10 illustrated in FIG. 1A, the die 16 is adhesively bonded face down to the die attach surface 22, with the bond pads on the die 16 aligned with the opening 26. Following the wire bonding step, an encapsulating resin 38 such as a Novoloc based epoxy, is molded onto the substrate 12 to encapsulate the die 16. In addition, a glob top 40 or other encapsulant can be formed over the wires 28 for protection. In some types of BGA packages the die 16 is attached back side down to the substrate 12, and the wire bonded wires 28 are encapsulated in the encapsulating resin 38.
The substrate 12 also includes a solder mask 20A formed on the conductor surface 24 and on the conductors 18. The solder mask 20A includes a pattern of via openings 25, wherein an array of solder balls 14 are located. During a solder ball bonding step, the solder balls 14 are bonded to ball bonding pads 31 on the conductors 18. Typically, solder ball bonding is performed by applying flux to the ball bonding pads 31, and to the solder balls 14. The solder balls 14 are then placed in the via openings 25, and the assembly is placed in an oven wherein the solder is reflowed to form a metallurgical solder bond. The solder mask 20A comprises an electrically insulating, low surface tension material, which prevents bridging of the solder material, and shorting between the solder balls 14 in the completed BGA package 10. In addition, the solder mask 20A helps to position the solder balls 14 for the solder reflow process.
Typically, the solder mask 20A comprises a photoimageable material, that can be blanket deposited as a wet or dry film, exposed through a mask, developed and then cured. Wet films are preferred because of their moisture resistance and low cost. Exposure and development of the solder mask 20A forms the via openings 25 in a required pattern and with required diameters. In addition, exposure and development of the solder mask 20A removes the mask material from the conductors 18 in a wire bonding area 36, wherein the wires 28 are wire bonded to the conductors 18.
In addition to the solder mask 20A being formed on the conductors 18, a solder mask 20B is also formed on the die attach surface 22. In general, the panel 30 is constructed with the solder mask 20B on the die attach surface 22 because the mask material is initially blanket deposited on all exposed surfaces of the panel 30 to form the solder mask 20A. For example, a spray coater or a curtain coater, can be used to blanket deposit the mask material on both the die attach surface 22, and on the conductor surface 24 of the substrates 12.
The presence of the solder mask 20B on the die attach surface 22 of the substrate 12 can cause problems in the BGA package 10. Firstly, the adhesive layer 34 which bonds the die 16 to the die attach surface 22 must be formed on the solder mask 20B. In general the solder mask 20B has a smooth surface, and a low surface tension. Accordingly, the adhesive bond between the die 16 and the solder mask 20B can be substandard. This can cause the die 16 to pop loose from the die attach surface 22.
Secondly, the solder mask 20B has hydrophilic properties, and tends to attract moisture. In order to drive off the moisture, along with solvents and other volatile compounds, a prebaking step can be performed on the solder mask 20B. However, this extra process step is sometimes not sufficient to prevent trapped moisture in the completed BGA package 10. Thirdly, the solder mask 20B can delaminate from the substrate 12 causing cracks to form in the BGA package 10.
In view of these and other deficiencies in conventional methods for fabricating BGA packages, improvements in BGA substrates, and in fabrication methods for BGA packages, are needed in the art.
In accordance with the present invention, an improved method for fabricating BGA packages, and an improved substrate for fabricating BGA packages, are provided.
The method, simply stated, comprises forming a substrate having a solder mask that substantially covers both major surfaces thereof, but which is patterned to leave a die attach area on the substrate open. The open die attach area permits a semiconductor die to be bonded directly to the substrate, rather than to the solder mask. This improves adhesion of the die to the substrate, reduces trapped moisture, and prevents delamination of the solder mask in the die attach area.
The substrate can comprise an electrically insulating material, such as bismaleimide triazine (BT). Initially, the substrate can be a segment of a panel which can be used to fabricate multiple BGA packages. The substrate includes a pattern of conductors formed on a first surface thereof, and a die attach area formed on an opposing second surface thereof. A first solder mask is formed on the first surface, and includes a pattern of via openings for attaching solder balls to ball bonding pads on the conductors. A second solder mask is formed on the second surface, and includes openings on the die attach area, permitting the die to be adhesively bonded directly to the substrate.
In an illustrative embodiment, the die is adhesively bonded face down to the substrate. In addition, bond pads on the die are placed in electrical communication with a corresponding pattern of conductors on the substrate, by wire bonding through openings in the substrate. Alternately, a flip chip process, or tape automated bonding, can be used to establish electrical communication between the die and the conductors.